Integrated circuits (“ICs”) are incorporated into many electronic devices. IC packaging has evolved, such that multiple ICs may be vertically stacked in so-called three-dimensional (“3D”) packages in order to save horizontal area on a printed circuit board (“PCB”). An alternative packaging technique, referred to as a 2.5D package may use an interposer, which may be formed from a semiconductor material such as silicon, for coupling one or more dies to a substrate. A plurality of IC chips, which may be of heterogeneous technologies, are mounted on the interposer. Connections among the various ICs are routed through conductive patterns in the interposer. These interposer and stacked IC techniques are called 2.5D-IC and 3D-IC, respectively. These packages are sometimes collectively referred to as “3D-IC.”
When IC chips are mounted onto an interposer wafer (before singulating the wafer), a flux generally comprising organic chemicals is applied onto the wafer before solder bumps are applied. The flux improves bump morphology during a solder bump reflow process. After the reflow process and before further packaging steps such as applying underfill, flux residues, particularly those in the narrow gaps between the chips and the wafer, are completely cleaned off. The flux residues can deteriorate package reliability and eventually performance of resulting devices. For example, flux residue comprising carboxylic acid can cause corrosion of the solder bumps.